1. Field of the Disclosure
The present disclosure relates to an automatic bias voltage control of an optical modulation device for use in multi Quadrature Amplitude Modulation (QAM). In particular, the present disclosure is suitable for the automatic bias voltage control of an optical modulation device that sends a quaternary or larger value QAM signal.
2. Discussion of the Background Art
As a transmission code for use in an optical transmission system, a QAM signal, capable of sending a large amount of optical signals at a low symbol rate has been paid attention. The simplest QAM is quaternary QAM and called Quadrature Phase Shift Keying (QPSK). The present disclosure is applicable to any multi-QAM modulators including QPSK, however for simplicity, a description will be mainly given of a 16 QAM system in the present disclosure. Here, some symbols have a bar placed on characters in figures, however   placed before the characters indicates that the symbols have the bar on the characters in the specification.
FIG. 1 shows the configuration of an optical modulation device in the related art. A continuous optical signal input to an IQ optical modulator M is divided into two signals by a first optical coupler 1 and input to the first and second optical modulation units which are an I-component optical modulation unit 2 and a Q-component optical modulation unit 3 (hereinafter first and second optical modulation units 2 and 3, respectively). The first and second optical modulation units 2 and 3 are generally composed of Mach-Zehnder Interferometer (MZI) type optical modulators and have the function of relatively changing an optical phase and optical intensity corresponding to the logic of the first quaternary data signals Data1 and  Data1 and that of second quaternary data signals Data2 and  Data2. Here, the relationship between the optical phase and optical intensity with respect to the four values of the data signals will be described below. In addition, the first and second bias voltages applied to the first and second optical modulation units 2 and 3, respectively, will be described below.
After the addition of a phase difference θ3 by an optical phase shifter 4 having an orthogonal bias electrode 101 to which a third bias voltage to be described below is applied, the outputs of the first and second optical modulation units 2 and 3 are multiplexed by the second optical coupler 5 and output as a 16 optical QAM signal. If θ3 is ±π/2, an ideal waveform can be obtained. It is equivalent to ¼ of a carrier wavelength, and general wavelength is of the order of micrometer, so then the adjustment must be performed really precisely. In addition, since the optical quality of an optical QAM signal is sensitive to an error in the optical phase shifter 4, it is really important to adjust the amount of phase shift induced by the optical phase shifter 4 to an appropriate value.
In general, this adjustment is made in such a manner that a third bias voltage (also called an orthogonal bias voltage) Vbias3 supplied from a third bias power supply 10 to the optical phase shifter 4 is adjusted. The optical phase shifter 4 is arranged on the rear stage of the second optical modulation unit 3 in FIG. 1, however it may be arranged on the rear stage of the first optical modulation unit 2, arranged on those of both the first and second optical modulation units 2 and 3, or arranged on the front stage. Hereinafter, for simplicity, it is assumed that the optical phase shifter 4 is arranged only on the rear stage of the second optical modulation unit 3.
Next, a description will be given of the relationship between the optical phase and optical intensity in the outputs of the first and second optical modulation units 2 and 3 with respect to the four values of the first and second quaternary data signals. As described above, it is general to use MZI type optical modulators as the first and second optical modulation units 2 and 3. The first and second optical modulation units 2 and 3 are driven by the first and second quaternary data signals, respectively. Each of these data signals is a quaternary NRZ Non Return-to-Zero (NRZ) signal. The first driving amplifier 6 amplifies the first quaternary data signal into the two complementary signals, and generates Data1 and  Data1. The second driving amplifier 7 amplifies the second quaternary data signal into the two complementary signals, and generates Data2 and  Data2.
The respective amplified quaternary data signals Data1 and  Data1 are applied to the two arms of the first optical modulation unit 2 via a first driving electrode 61 to generate a phase shift ±φ1. The respective amplified quaternary data signals Data2 and  Data2 are applied to the two arms of the second optical modulation unit 3 via a second driving electrode 71 to generate a phase shift ±φ2. The values of the phase delays φ1 and φ2 change with the four values of the respective data signals. In addition, DC voltages (data bias voltages) Vbias1 and V′bias1 are generated by a first bias power supply 8, and optical phase shifts +θ1 and −θ′1 are further added via a first bias electrode 81. Moreover, DC voltages (data bias voltages) Vbias2 and V′bias2 are generated by a second bias power supply 9, and optical phase shifts +θ2 and −θ′2 are further added via a second bias electrode 91.
Next, the expression of the various voltages described above is defined as follows. The four signal levels of a differential signal (Data1-  Data1) generated by the first driving amplifier 6 are expressed as V0, V1, −V1, and −V0, and it is assumed that V0>V1>−V1>−V0 is established. Since the optical characteristics of the first optical modulation unit 2 are generally the same as those of the second optical modulation unit 3, the four signal levels of a differential signal (Data2-  Data2) generated by the second driving amplifier 7 are also expressed as V0, V1, −V1, and −V0.
Vbias1, V′bias1, Vbias2, and V′bias2 select the null points of the first and second optical modulation units 2 and 3. In other words, the output light of the first and second optical modulation units 2 and 3 is set to extinguish when the differential voltages of the signals generated by the first and second driving amplifiers 6 and 7 are 0. In addition, the maximum amplitudes of the differential outputs of the first and second driving amplifiers 6 and 7 are set so as not to exceed twice the half wavelength voltages Vπ of the first and second optical modulation units 2 and 3. Accordingly, 2Vπ≧V0−(−V0)=2V0 is established.
Here, a description will be given, with reference to FIG. 1, of the half wavelength voltage Vπ of the first optical modulation unit 2. The first optical modulation unit 2 is a MZI type modulator and incorporates two waveguides. It is assumed that a data bias is adjusted to extinguish the output of the first optical modulation unit 2 when both of the voltages Data1 and  Data1 applied to the two waveguides are 0. In general, complementary driving signals are applied to the two waveguides, and 2Vx is called the half wavelength voltage Vπ of the first optical modulation unit 2 if the optical output of the first optical modulation unit 2 reaches maximum intensity under Data1=Vx and  Data1=−Vx. The optical output of the first optical modulation unit 2 reaches the maximum intensity even under Data1=−Vx and  Data1=Vx, however the optical phase of the optical output in this case is made different by π compared with the former example. Since the first optical modulation unit 2 changes an optical phase by making use of the property, each of Data1 and  Data1 is set to have an amplitude of 2Vx=Vπ at the maximum and (Data1-  Data1) is set to have an amplitude of 2Vπ at the maximum. The second optical modulation unit 3 is configured in the same manner as the first optical modulation unit 2.
Here, in the IQ optical modulator M shown in FIG. 1, each of the driving signal electrodes is configured to apply positive and negative complementary voltages to the two waveguides, and thus four electrodes in total exist. The IQ optical modulator M of this type is called a dual drive type. On the other hand, a single drive type IQ optical modulator M has only two driving signal electrodes. In such a configuration, the first driving electrode 61 simultaneously applies electric fields to the two waveguides inside the first optical modulation unit 2, and the second driving electrode 71 simultaneously applies electric fields to the two waveguides inside the second optical modulation unit 3. With the anisotropy of the four waveguides, the single drive type IQ optical modulator M can realize the same function as that of the dual drive type. Also in this configuration, the quaternary data signals applied to the first and second driving electrodes 61 and 71 are the four voltages V0, V1, −V1, and −V0, and the amplitude of each of the driving signals is set so as not to exceed twice the half wavelength voltage Vπ.
FIG. 2 shows the characteristics of the optical modulation device in a case in which no bias drift occurs. FIG. 2 shows the relationship between the electric field E1 of the output light of the first optical modulation unit 2 and V0, V1, −V1, −V0, and Vbias1, and the relationship between the electric field E2 of the output light of the second optical modulation unit 3 and V0, V1, −V1, −V0, and Vbias2 is also expressed just like FIG. 2. When the sum of the potentials V0, V1, −V1, and −V0 of a driving signal and the data bias Vbias1 is shown in the horizontal axis and the electric field E1 of the output light is shown in the vertical axis, a sine wave is drawn. When the optimum data bias is applied, V0, V1, −V1, and −V0 are arranged symmetrically to the null point and the electric fields E11, E12, E13, and E14 of the output light generated by V0, V1, −V1, and −V0 are also arranged symmetrically to the 0 level.
FIG. 3 shows a constellation in a case in which no bias drift occurs. Since θ3=π/2 is established when optimum Vbias3 is applied to the optical phase shifter 4 and the optical phase of the output light of the first optical modulation unit 2 and that of the second optical modulation unit 3 are kept to be orthogonal to each other, the constellation of the output light of the IQ optical modulator M is one having a lattice pattern as shown in FIG. 3. Here, it is important that the respective symbols of the constellation are arranged symmetrically to the origin. This symmetry is a property common to QAM in addition to 16QAM. Here, since E1 and E2 are orthogonal to each other when θ3=π/2 is kept, an In-Phase component and a Quadrarure-Phase component are sometimes briefly called an I-component and a Q-component, respectively.
The optical power Ptotal of a 16QAM signal is proportional to the sum of the square of the electric field of each of the symbols of the constellation. Ptotal is expressed by Equation 1.Ptotal∝Σ(|E1K|2+|E2L|2)  (Equation 1)
Here, K and L have the sum of 1 to 4.
In a case in which Vbias1 and Vbias2 are kept at optimum values even if a bias drift occurs in Vbias3 and θ3 has a value different from π/2, the total of the optical power Ptotal does not change as described in detail in Non-Patent Literature 1. Therefore, it is relatively difficult to detect a drift occurring in Vbias3. However, it becomes possible to detect a drift occurring in Vbias3 with the application of dithering to Vbias1 and Vbias2 using the technique of asymmetric bias dithering described in Non-Patent Literature 1.
Next, consideration will be given to a case in which a bias drift occurs in the data bias Vbias1 or Vbias2. In the case of quaternary QAM, i.e., a QPSK signal, since the optical power of modulation light immediately decreases after a bias drift occurs in the data bias Vbias1 or Vbias2, it is relatively easy to detect the bias drift occurring in the data bias Vbias1 or Vbias2. Thus, it becomes possible to detect and correct bias drifts occurring in all the Vbias1, Vbias2, and Vbias3 of the QPSK modulator with the technique of the asymmetric bias dithering described above.